The traditional boost PFC circuits are widely used due to their unique features of: simplicity, capability of boosting the voltages, and relatively higher efficiency. Following that the relatively higher power levels of the power sources are more and more desired, several of these modulated boost PFC circuits are required to be coupled in series or in parallel usually. For example, three-level boost PFC circuits are usually employed to let the voltage stresses of the switches be limited to a half of the output voltage in the high voltage applications. In which, apparatuses respectively composed of several boost converters and coupled in parallel are usually employed. Please refer to FIG. 1, it is the schematic diagram of the dual boost PFC circuit in the prior art. In which, two boost units are coupled to the bridge rectifier circuit located at the input side. Compared with the traditional boost converters, the alternatively controlled dual boost PFC circuits have the following advantages: 1. the current level of each unit can be decreased to a-half, and 2. the ripples of the AC current and the output current can be decreased. On the other hand, the configurations of this kind of circuits have their problems: 1. the configurations of the circuits are relatively more complex, 2. the currents of the two sub-circuits are unbalanced due to the discrepancies of the two sets of elements. This phenomenon is relatively more serious when the whole apparatus is operated under the inductor current continuous conduction mode (CCM).
Due to the above-mentioned problems, a simple and effective controlling method for accomplishing the current sharing between the two sub-circuits, controlling the total inductor current, and tracking the output voltage of the bridge-rectifier circuit is required for this kind of circuits. The existing solutions for the aforementioned problem in the prior arts are analyzed as follow.
A dual boost PFC circuit has to guarantee that the total inductor current iL and the rectifying voltage have the same phase such that the AC line current and the AC input voltage could have the same phase, and the current sharing between the two sub-circuits has to be guaranteed also. Thus, there are certain difficulties for this kind of apparatuses to properly accomplish the controlling of the circuit. Firstly, it is the sampling of the current. If the total inductor current iL is sampled, the total inductor current is easy to control, but the balance between the two sub-circuits is not guaranteed. Otherwise, if the inductor current of each sub-circuit is sampled, the current sharing could be guaranteed, but other measures are required to keep the total inductor current and the rectifying voltage at the same phase. Among the traditional controlling methods for the dual boost PFC circuits, the method proposed in “Multiple parallelized boost PFC devices” (China Patent no. CN1353497), which samples the total inductor current and controls the circuit by the power factor control IC UC3854 to make sure that the total inductor current and the rectifying voltage have the same phase. Meanwhile, the output control signal of the UC3854 is divided into two driving signals having a phase-shift of 180 degrees through frequency dividing, which respectively control two sets of main power switches, and let the two sets of power switches conduct alternately. The unique feature of this controlling method is that the total inductor current is being controlled, the AC line current and the AC input voltage have the same phase, and the control circuit is simplified. But this control method does not accomplish the real current sharing, the current difference between the two sub-circuits are not eliminated by this method since there exists current unbalancing between the two sub-circuits due to the discrepancies of the two sets of elements (power switches, boost inductors, output diodes, control circuit dividing elements etc.) of the two sub-circuits. Finally, this will cause the power switch having relatively larger current being over-heated, the boost inductor being saturated, and the efficiency of the system being decreased.
There is another representative controlling method in the prior art, which is proposed by Laszlo Balogh and Richard Redl in “Power-Factor Correction with Interleaved Boost Converters in Continuous-Inductor-Current Mode,” Proceedings of APEC' 93, pp. 168–174. A UC3854 control IC is employed to control the total inductor current, to detect the current of each power switch, employ the differential amplifying to amplify the currents of the two sub-circuits, decide the slopes of the two saw-tooth waves for controlling the two pulse signals by the output of the differential amplifier, and accomplish the current sharing. This control method samples the total inductor current, detects the currents of the two sub-circuits, and employs extra peripheral circuits for producing the alternately controlling signals such that the whole control circuit becomes relatively complex due to the total number of elements is increased.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the current sharing method and apparatus for alternately controlling the parallel connected boost PFC circuits are finally conceived by the applicants.